Is this legal or ethical?

I travel a lot since my work requires it. I have several properties in different areas. One time I have to sell one vacant lot in Chicago since I am an out of state owner. I made a deal to sell my vacant lot. The buyer was a licensed real estate agent who within a week had sold it to someone else for $50,000 more than what they paid me. Is this legal or ethical? Can I do something about it?